Each component of the belt contributes to increasing the performance in terms of mechanical resistance, so as to decrease the risk of failure of the belt and to increase the specific transmissible power.
Cords especially contribute to ensuring the required mechanical characteristics for the belt and substantially contribute to the modulus of the belt itself and therefore specifically ensure the maintenance of the performance of the belt in the course of time. Cords are generally formed by twisting high-modulus fibres several times.
Cords are also normally treated with substances adapted to increase the compatibility of the fibres with the body compound that surrounds the cords themselves.
For example, the cords may be treated with elastomeric latexes that serve as “adhesives”.
The body compound allows connecting the various elements and must have appropriate hardness and ensure that the various elements forming the belt itself synergically contribute to the final performance of the belt itself.
The body compounds contain one or more elastomeric materials possibly enriched with fibres to increase the hardness thereof. The coating fabric of the belts increases the resistance to abrasion and hence protects the working surface of the belt from wear due to rubbing between the sides and the tops of the belt teeth and the sides and the bottoms of the races of the pulley with which the belt interacts.
Furthermore, the coating fabric reduces the coefficient of friction on the working surface, reduces the deformability of the teeth and especially reinforces the root of the tooth thereby avoiding the failure thereof.
The coating fabric used may consist of a single layer or, alternatively, may be double-layered so as to ensure a greater sturdiness and stiffness.
The fabric is normally treated with an adhesive, for example resorcinol and formaldehyde latex (RFL) to increase adherence between the body and the fabric itself.
Moreover, a number of methods are employed to increase the resistance to wear of drive belts by modifying the structure of the coating fabric or performing different treatments on the fabric, for example, treatments where the fabric is treated with halogenated polymers, for example PTFE treatments.
Said treatments do not, however, lead to any great increase in resistance to wear as the coating fabric of the toothed belt, after a short time of use, again becomes the working surface and accordingly the surface exposed to wear.
Alternatively, treatments (also designated as adhesive compositions) for toothed belt fabrics including an anti-friction material selected from the group consisting of molybdenum sulphide and graphite are also known from patent applications JP2001304343 and JP2001208137 to MITSUBOSHI. However, said treatments are also applied to the threads of the fabric as a glue and do not form a separate layer and therefore these anti-friction materials also do not allow to obtain an optimal resistance to wear.
As a matter of fact, also in these cases the fabric is the actual working surface of the toothed belt, i.e. the surface that in use is in direct contact with the pulleys, and therefore also in this case the treatment allows to obtain only a slight increase of the resistance to abrasion and the belt is also very noisy, especially in the step of engaging the pulley.
To overcome such a problem it has been suggested to radically change the structure of the belt in patent EP1157813 in the name of the same applicant, where it is suggested to cover the coating fabric with a resistant layer comprising a fluorinated plastomer, an elastomeric material and a vulcanisation agent. The fluorinated plastomer is present in the resistant layer in a larger amount with respect to the elastomeric material so as to greatly facilitate the formation of an actual layer which is separate from the fabric.
The layer is distinct from the fabric, does not permeate in the fabric and is advantageously applied to the fabric by means of a calendering step. An adhesive layer is advantageously present between the fabric and the resistant layer.
The use of the resistant layer has allowed to obtain good results in terms of the increase in the resistance to wear, as the working surface of the belt is in this case formed by the resistant layer itself and not by the fabric, which has shown to be particularly resistant to wear and provides for a lower noisiness during the step of engaging the teeth of the belt on the pulley.
The resistant layer however has some drawbacks. Specifically, said layer comprises a fluorinated plastomer including particles which have a mean size of 20 μm or more and are in the form of agglomerates. Such agglomerates therefore have a size such as to imply a difficult miscibility with the elastomer in solution. The agglomerates are therefore also present in the final resistant layer which is therefore heterogeneous and such heterogeneity may generate a high noisiness during the use of the belt.
A further drawback consists in that the fluoropolymer is very expensive and a consistent addition of such a polymer results poorly cost-effective.
Alternative solutions are continuously sought for obtaining a coating layer for toothed belts allowing to provide a high resistance to wear in the whole range of operating temperatures of the belt and a low noisiness at low and high speeds at the same time.